ABSTRACT The use of umbilical cord blood (CB) as a source of hematopoietic stem cells (HSCs) for transplantation therapy is rapidly expanding, particularly for the treatment of acute myeloid leukemia. However, CB-derived HSCs are of limited use in adults because the number of HSCs available in CB is not sufficient; thus, ex vivo expansion of HSCs is needed to bridge this gap in availability. Activation of p38 mitogen-activated protein kinase (p38 MAPK or p38) has been implicated in the pathogenesis of a variety of hematopoietic disorders, including Franconi anemia (FA) and myelodysplastic syndromes (MDS). Furthermore, oxidative stress, mutation of ATM, and loss of FoxO3 all activate p38 MAPK through up-regulation of reactive oxygen species (ROS) and lead to premature exhaustion of HSCs. These studies suggest that activation of the ROS-p38 pathway may negatively affect HSC self-renewal and expansion. Interestingly, inhibition of p38 has been shown to improve the function of HSCs and hematopoietic progenitor cells (HPCs) in FA and MDS patients and to rescue p38 activation-mediated premature exhaustion of HSCs in FoxO3-/- mice. However, the role of the ROS-p38 MAPK pathway in the regulation of human HSC self-renewal and ex vivo expansion is unclear. Our preliminary studies support the hypothesis that targeted inhibition of the ROS-p38 pathway enhances ex vivo expansion of HSCs. The goal of this project is to develop a novel and innovative approach to increase CB HSC ex vivo expansion by targeting the ROS-p38 pathway. Aim 1 will validate the hypothesis that inhibition of the p38 pathway promotes ex vivo expansion of CB HSCs, and Aim 2 will elucidate the mechanisms by which p38 regulates HSC expansion. The results of this study will provide novel insights into the role of the ROS-p38 pathway in the regulation of HSC self-renewal and differentiation. Such information will be essential to the development of innovative culture strategies for the ex vivo expansion of HSCs.